1. Field of the Invention
The present invention relates to a photo detecting method for eliminating a pulse noise caused by a pulse bias circuit and, more particularly, to a noise elimination periodically applied to a photo detecting device, and also to a photo receiving circuit using the noise elimination method.
2. Description of the Related Art
In the field of photon detection, an avalanche photodiode (hereinafter, referred to as APD) is generally used as an element for detecting a single photon. A basic photon detection technique is as follows. A reverse-bias voltage not less than an APD breakdown voltage (VBd) is applied to an APD so that the multiplication factor of the APD is increased to an extremely large value, whereby a photocurrent triggered by a single photon is amplified to such an extent that the signal amplitude becomes large enough for an external circuit to be able to process the signal.
As is well known, once an APD to which a reverse-bias voltage not less than VBd is applied is broken down and a multiplication current flows, this breakdown state continues regardless of which of a photon signal and a dark noise has caused the breakdown. Therefore, in general, a reverse-bias voltage not less than VBd is periodically applied in the form of pulses, in synchronization with the timing when a photon is expected to arrive at the APD. Such a reverse-bias pulse to be applied to the APD defines periodicity by which the reverse-bias voltage decreases below VBd when clearly no photon signal is expected to arrive. Thus, the breakdown state can be stopped.
However, applying the reverse-bias voltage in the form of a pulse to an APD causes a differential-waveform signal having a waveform determined according to the parasitic capacitance of the APD device and a change over time in the voltage of the reverse-bias pulse. When this differential-waveform signal is outputted, a signal decision circuit at a subsequent stage may erroneously decide that it is a photon-detection signal, which becomes a cause of degrading its ultimate signal error rate. Accordingly, there have been proposed several countermeasures against pulse noises.
For example, when a reverse-bias pulse is simultaneously applied to two APDs, these APDs each output differential-waveform signals having the substantially same differential waveform. Therefore, these differential-waveform signals can be eliminated from the outputs of the two APDs through a subtraction circuit employing a hybrid coupler (see Kosaka, H. et al. “Single-photon interference experiment over 100 km for quantum cryptography system using balanced gated-mode photon detector” ELECTRONICS LETTERS, Aug. 7, 2003, Vol. 39, No. 16, p. 1200).
Moreover, according to Prochazka, I. “Peltier-cooled and actively quenched operation of InGaAs/InP avalanche photodiodes as photon counters at a 1.55-μm wavelength” APPLIED OPTICS, Vol. 40, No. 33, pp. 6012 to 6018 (Nov. 20, 2001), a capacitor equivalent to the parasitic capacitance of an APD is arranged in parallel and the same pulse bias is applied to the APD and the capacitor (see FIG. 2). In this arrangement, the capacitor outputs a pulse noise having the same waveform as the output of the APD. Accordingly, these pulse noise outputs can be eliminated through a subtraction circuit employing an operational amplifier.
As described above, the use of the above-described subtraction circuit can prevent the decision circuit at the subsequent stage from erroneously deciding a pulse noise to be a photocurrent signal and therefore makes it possible to carry out decision only on a photocurrent signal from the APD.
However, the above-described conventional pulse noise elimination methods require a subtraction circuit such as a hybrid coupler or an operational amplifier, and therefore it has been difficult to achieve enhanced speed. A hybrid coupler is a circuit employing a plurality of coils and, because of its physical structure and manufacture precision, has difficulty in achieving higher speed as well as a wider band. For example, in the case of a photon detector using a reverse-bias pulse with a pulse width of one nanosecond, an output signal from the APD also has a pulse width of about one nanosecond, which means that a signal bandwidth of around one GHz is required.
The problem with enhancing speed is not limited to the hybrid coupler-based circuitry but similarly exists in electronic circuit-based circuitry using an operational amplifier as a subtraction circuit. This is because, in the case of the electronic circuit, the processing by the subtraction circuit, which is required for photon detection, is common-mode processing. The common-mode processing, in a general design technique for high-speed circuits, tends to deteriorate in performance as the bandwidth increases. For a commercially available operational amplifier, this performance is generally provided as a common-mode rejection ratio (CMRR) on a data sheet.
In addition, according to the above-described conventional pulse noise elimination methods, pulse noise elimination is performed by analog subtraction of signal waveforms using a subtraction circuit such as a hybrid coupler or an operational amplifier. Therefore, these methods are based on the precondition that the pulse noises outputted from, for example, two APDs have the same shapes. Pulse noises cannot be completely eliminated if the characteristics of two APDs and peripheral circuits are different from each other. Accordingly, it is necessary that the characteristics of the pair of APDs be made equal.
Differences in the above-mentioned characteristics include: for example, a difference in pulse noise amplitude due to a difference in parasitic capacitance between APD devices; a difference in quantum efficiency; a difference in amplitude between reverse-bias pulses applied; the non-linearity of an amplifier, as one of the results of electronic circuit implementation; differences in gain and loss; a difference in waveform disturbance due to signal reflection; and so on. There is a good possibility that these differences may cause a difference between the pulse noise waveforms outputted from two APDs. To solve this problem of differences, selection work is required to make the device characteristics of APDs equal. In addition, higher precision in implementation is also required. These requirements lead to an increase in cost and a decrease in yield.